Armchair economist here. Is it correct to say that a country like Italy, with its low productivity, constantly needs to devalue its currency to keep its production competitive; and that while doing so would effectively lower the wealth of the country, it could still keep an apparent growth of the salaries, so that there would be an incentive to internal consumption? And that what happens in Italy instead is that the wages need to shrink, which causes (a state close to) deflation, which causes even more saving and depresses internal consumption?

- Economic homogeneity across an entire nation state can only reasonably expected with small countries

Well, that is not true and probably not an issue. Every country has automatic mechanisms of wealth redistribution across its regions: California feeds most of the central US states. But the EU is a monetary unit without a proper redistribution mechanism between its regions: it's hard to convince the Germans that their tax money should subsidize Italy because it's part of a national unity.